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Sea surface temperature (SST) is characterized by abundant warm and cold structures that influence the overlying atmospheric boundary layer dynamics through two different mechanisms. First, turbulence and large eddies in the lower troposphere are affected by atmospheric stability, which can be modified by local SST, resulting in enhanced vertical mixing and larger surface winds over warmer waters. Second, the thermodynamic adjustment of air density to the underlying SST structures and the subsequent changes in atmospheric pressure drive secondary circulations. This paper aims to disentangle the effects of these processes and explore the environmental conditions that favor them. Two main environmental variables are considered: the large-scale air–sea temperature difference (proxy for stability) and wind speed. Using 5 years of daily reanalyses data, we investigate the 10-m wind response to SST structures. Based on linear regression between wind divergence and SST derivatives, we show that both mechanisms operate over a large spectrum of conditions. Ten-meter wind divergence is strongly impacted by the local SST via its effect on vertical mixing for midwind regimes in slightly unstable to near-neutral conditions, whereas the secondary circulation is important in two distinct regimes: low wind speed with a slightly unstable air column and high background wind speed with a very unstable air column. The first regime is explained by the prolonged Lagrangian time that the air parcel stays over an SST structure while the second one is related to strong heat fluxes at the air–sea interface, which greatly modify the marine atmospheric boundary layer properties. Location and frequency of the environmentally favorable conditions are discussed, as well as the response in low-cloud cover and rainfall. Significance Statement The main objective of this study is to explore the wind response to thermal structures at the sea surface under different environmental conditions using the latest atmospheric reanalysis. Recent literature suggests that fine-scale air–sea interactions affect a large spectrum of atmospheric dynamics, from seasonal to weather-type regimes. It is thus important to characterize the atmospheric response to ocean surface variability. Our findings describe the environmental conditions for which the two main physical processes through which the atmosphere responds to sea surface temperature structures are active the most and can guide the development of high-resolution observing missions and campaigns in specific geographical locations and seasons to retrieve data that can be used to improve parameterization in models.

Aerosols play a crucial role in climate through different feedback mechanisms, affecting radiation, clouds and air column stability. This study focuses on the altitude-dependence of the cloud mediated indirect effects of aerosols in the Great Alpine Region (GAR), an area characterised by high pollution levels from anthropic activities in the Po Valley and a complex orography with the highest mountains in Europe. Using a regional atmospheric model, 5-years long convective-permitting sensitivity experiments have been run with different surface aerosol fluxes. The results show that seasonal mean cloud cover, temperature, and precipitations are affected by the aerosol concentrations in the air column, and that the response to pollution is both elevation and season dependent. The overall cloud cover increase with aerosol levels leads to either surface cooling or warming depending on the surface albedo (snow covered or not). Furthermore, different types of clouds have a different sensitivity to aerosols: while the lifetime of low pressure system clouds and orographic clouds is generally increased at high levels of aerosols, convective clouds (typical of the summer season) can actually decrease at high levels of pollution, due to the reduction of strong updrafts.

Thermal structures at the sea surface are known to affect the overlying atmospheric dynamics over various spatio-temporal scales, from hourly and sub-kilometric to annual and O(1000 km). The releva...

Using 25years of ERA5 reanalysis data, this study shows that wind divergence is partially driven by small-scale sea surface temperature (SST) patterns via their effect on the boundary layer stability. Moreover, strong warm-to-cold fronts (the upper quartile) are associated with a mean increase of cloud cover of 10%±5% and a mean increase in the probability of a rain event of 15%±6%, with respect to the average values. The cloud and rainfall dependence on SST fronts is more pronounced in Fall, probably due to the stronger SST gradients present at the end of the summer season.

Sea Surface Temperature (SST) is known to affect the marine atmospheric boundary layer (MABL) at scales smaller than O(1000 km) via different mechanisms. In particular, the oceanic thermal forcing induces modification in the wind speed, its divergence and its curl by the action of the Downward Momentum Mixing (DMM) mechanism and the Pressure Adjustment (PA) one. By analyzing 25 years of observations of surface wind speed and SST in the Mediterranean, it is found that the probability of observing surface wind convergence is significantly higher over a thermal oceanic front crossed from the warm to the cold side, in agreement with the DMM mechanism. Physically, this is due to a deceleration of the surface wind over the cold side of the SST front because of the increased atmospheric stability over the cold water. The strongest response in terms of surface convergence is found when atmospheric fronts (already characterized by strong surface convergence) cross SST gradients from the warm to the cold side.Using 25 years of ERA5 reanalysis data, it is also found that the wind divergence variability within the MABL (until about 925 hPa) is partially driven by mesoscale SST patterns via their effect on the boundary layer stability. This results in a cloud cover and rainfall response: when a wind blows from warm-to-cold (cold-to-warm) ocean patterns, a converging (diverging) cell is enhanced, increasing (decreasing) low-cloud cover and favouring rainfall. Specifically, strong warm-to-cold fronts (the upper 25th percentile) are associated with a mean increase of cloud cover of 10±5% and a mean increase in the probability of a rain event of 15±6%, with respect to the average values. The cloud and rainfall dependence on SST fronts is more pronounced in fall than in the rest of the year, probably due to the stronger SST gradients present at the end of the summer season. The effects on cloud cover, in particular, are a preferential way through which mesoscale SST structures can impact the radiation budget and, thus, the Earth climate.

Surface winds (10 m equivalent neutral wind velocity) from scatterometer missions since 1992 to present require homogenization to meet the requirements for oceanic and atmospheric climate data records. Sources of differences between winds retrieved from different scatterometer measurements mainly arise from calibration/validation procedures used for each scatterometer and differences in measurement physics. In this study, we focus on the calibration/validation component of the European Remote Sensing Satellite (ERS)-1 and ERS-2 wind speed biases. ERS-1 and ERS-2 data, named as WNF products, are from the Institut Francais de Recherche pour l'Exploitation de la MER (IFREMER). In addition to WNF data, the newly calibrated ERS-2 products provided by the European Space Agency (ESA), indicated as ASPS2.0 products, are also used. Our approach utilizes collocated satellite-buoy data. Expected values of the normalized radar cross section (NRCS) are calculated from buoy winds for each antenna beam using the Cmod5.n geophysical model function. The comparisons between expected and measured NRCS examine differences along with variables such as backscatter coefficient and incidence angle ranges. The difference between the expected and measured NRCS is then used to set up empirical models aiming at the correction for biases in ERS-1 and ERS-2 WNF NRCS calibrations. Finally, ERS-1 and ERS-2 wind retrievals are reprocessed using the corrected NRCS and Cmod5.n. These earlier corrected ERS-1/2 winds are analysed along with later scatterometer data (QuikSCAT and ASCAT-A) for their deviations from in situ buoy winds during 1992–2011 period. The scatterometer data homogeneity is also investigated at global scales based on the use of collocated scatterometer retrievals and atmospheric re-analyses winds derived from ERA Interim and CFSR models.

Satellite scatterometers provide continuously valuable surface wind speed and direction estimates over the global ocean on a regular grid both in space and time. The Level 3 data derived from the Advanced Scatterometer (ASCAT), available at 1/4° spatial resolution (hereafter AS25), and Quick Scatterometer (QuikSCAT), available on 1/2° and 1/4° horizontal grids (QS50 and QS25, respectively), are studied at regional scales in both the Benguela and Canary upwelling systems. They are compared to the European Center for Medium-Range Weather Forecast surface wind analysis, with insight into their intrinsic and actual spatial resolutions. In the coastal band, the finest spatial patterns are found in the QS25 winds and are O(75 km). This demonstrates the sensitivity of the high-resolution satellite-derived winds to coastal processes related to sea surface temperature (SST) perturbations and land-sea transition. Next, short-lived upwelling episodes (SUEs) calculated from SST anomalies are defined consistently with the QS25 actual resolution. These cold events refer to local, short-lived perturbations that add to seasonal upwelling variability. We characterize concomitant atmospheric synoptic conditions for SUEs identified at chosen latitudes and highlight two subregions in both upwelling systems, with contrasted patterns for the alongshore wind stress component and curl. The complexity of the latter patterns is closely linked to local, short-term SST variability. Closer to the shore, numerical sensitivity experiments show that the imbalance between Ekman transport and Ekman pumping has an impact on ocean dynamics: wind reduction in the coastal QS25 forcing, partially induced by orography, tends to reduce coastal SST cooling.